Is this a memory leak?

Well I'm not sure if this is a memory leak or not. I'm pretty certain it is, but need the view of a more experienced programmer .

Code:

// for example 
char* get_buffer()
{
	char* buffer = malloc(20 * sizeof(char));
	// etc
	return buffer;	// memory leak?
}

At first glance (to me at least) it looks like a memory leak because the buffer can apparently not be free'd.

How would I get around this without the use of gloabal variables?

You need to make sure your caller frees it.

Code:

getbuffer();
getbuffer();
getbuffer();
getbuffer();
getbuffer();
getbuffer();
getbuffer();
getbuffer();

This is a memory leak.

Code:

printf( "%s", getbuffer() );

This is a memory leak as well.

Code:

char* lpszText = NULL;

lpszText = getbuffer();

// do something with lpszText

if( lpszText != NULL ) 
{
     free( lpszText );
     lpszText = NULL;
}

This is not.



We have a ton of functions like this. It's easy to make mistakes using them, but they are better than the alternative of either strings ( sl-o-o-o-w for 100% text-processing applications ) or fixed size arrays ( always either too small or way too large ).

I've got it thanks for your help

Hi,

I am pretty new to C++. What I think about the code is that the "get_buffer" function returns nothing. Because the pointer is a local variable of the function it will be freed as soon as it goes out of scope (that is the end of the "get_buffer" function call). Am I getting some(serious) things wrong here?

Originally Posted by sparks

Hi,

I am pretty new to C++. What I think about the code is that the "get_buffer" function returns nothing. Because the pointer is a local variable of the function it will be freed as soon as it goes out of scope (that is the end of the "get_buffer" function call). Am I getting some(serious) things wrong here?

Damn! Beat me to it. Yeah, never return a pointer...

There are circumstances where you will return a pointer to something. It is quite legal to do so and I do it in applications.

Returning a pointer from a function is fine... if the memory it points to is either that of a static object or dynamically allocated and not simply local to the function (and if dynamically allocated you should make sure the caller free/deletes the memory).

Originally Posted by Berticus

Damn! Beat me to it. Yeah, never return a pointer...

So what's really the problem to that piece of code ? Are we saying the "Super Moderator" didn't point out the right problem and I got it right ? I really wouldn't expect that as a freshman to C++ though.

Originally Posted by sparks

Hi,

I am pretty new to C++. What I think about the code is that the "get_buffer" function returns nothing. Because the pointer is a local variable of the function it will be freed as soon as it goes out of scope (that is the end of the "get_buffer" function call). Am I getting some(serious) things wrong here?

This is incorrect. The pointer is returned by value, meaning a copy of the pointer's value is returned and stored in whatever variable (if any) accepts it in the calling code. The pointer points at dynamic memory, which is not local to the function. So the dynamic memory still exists at the location stored by the pointer, even after the function exits. The allocated memory is still valid, and the location of that memory stored in the pointer can be returned from the function.

Originally Posted by Daved

This is incorrect. The pointer is returned by value, meaning a copy of the pointer's value is returned and stored in whatever variable (if any) accepts it in the calling code. The pointer points at dynamic memory, which is not local to the function. So the dynamic memory still exists at the location stored by the pointer, even after the function exits. The allocated memory is still valid, and the location of that memory stored in the pointer can be returned from the function.

Thanks Daved for correcting me. I got the idea completely wrong.